Electrochromic devices, and electrochromic media suitable for use therein, are the subject of numerous U.S. patents, including U.S. Pat. No. 4,902,108, entitled "Single-Compartment, Self-Erasing, Solution-Phase Electrochromic Devices, Solutions for Use Therein, and Uses Thereof", issued Feb. 20, 1990 to H. J. Byker; Canadian Pat. No. 1,300,945, entitled "Automatic Rearview Mirror System for Automotive Vehicles", issued May 19, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799, entitled "Variable Reflectance Motor Vehicle Mirror", issued Jul. 7, 1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled "Electro-Optic Device:, issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No. 5,204,778, entitled "Control System For Automatic Rearview Mirrors", issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled "Tinted Solution-Phase Electrochromic Mirrors", issued Jan. 11, 1994 to D. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled "UV-Stabilized Compositions and Methods", issued Jan. 18, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "Variable Reflectance Mirror", issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No. 5,294,376, entitled "Bipyridinium Salt Solutions", issued Mar. 15, 1994 to H. J. Byker; U.S. Pat. No. 5,336,448, entitled "Electrochromic Devices with Bipyridinium Salt Solutions", issued Aug. 9, 1994 to H. J. Byker; U.S. Pat. No. 5,434,407, entitled "Automatic Rearview Mirror Incorporating Light Pipe", issued Jan. 18, 1995 to F. T. Bauer et al.; U.S. Pat. No. 5,448,397, entitled "Outside Automatic Rearview Mirror for Automotive Vehicles", issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat. No. 5,451,822, entitled "Electronic Control System", issued Sep. 19, 1995 to J. H. Bechtel et al., each of which patents is assigned to the assignee of the present invention and the disclosures of each of which are hereby incorporated herein by reference, are typical of modern day automatic rearview mirrors for motor vehicles. These patent references describe electrochromic devices, their manufacture, and electrochromic compounds useful therein, in great detail.
While numerous electrochromic devices are possible, the greatest interest and commercial importance are associated with electrochromic windows, light filters and mirrors. A brief discussion of these devices will help to facilitate an understanding of the present invention.
Electrochromic devices are, in general, prepared from two parallel substrates coated on their inner surfaces with conductive coatings, at least one of which is transparent such as tin oxide, or the like. Additional transparent conductive materials include fluorine doped tin oxide (FTO), tin doped indium oxide (ITO), ITO/metal/ITO (IMI) as disclosed in "Transparent Conductive Multilayer-Systems for FPD Applications", by J. Stollenwerk, B. Ocker, K. H. Kretschmer of LEYBOLD AG, Alzenau, Germany, and the materials described in above-referenced U.S. Pat. No. 5,202,787, such as TEC 20 or TEC 15, available from Libbey Owens-Ford Co. (LOF) of Toledo, Ohio. Co-filed U.S. patent application entitled "AN IMPROVED ELECTRO-OPTIC DEVICE INCLUDING A LOW SHEET RESISTANCE, HIGH TRANSMISSION TRANSPARENT ELECTRODE" describes a low sheet resistance, high transmission, scratch resistant transparent electrode that forms strong bonds with adhesives, is not oxygen sensitive, and can be bent to form convex or aspheric electro-optic mirror elements or tempered in air without adverse side effects. The disclosure of this commonly assigned application is hereby incorporated herein by reference.
The two substrates of the device are separated by a gap or "cavity", into which is introduced the electrochromic medium. This medium contains at least one anodic or cathodic electrochromic compound which changes color upon electrochemical oxidation or reduction, and at least one additional electroactive species which may be reduced or oxidized to maintain charge neutrality. Upon application of a suitable voltage between the electrodes, the electroactive compounds are oxidized or reduced depending upon their redox type, changing the color of the electrochromic medium. In most applications, the electroactive compounds are electrochromic compounds which change from a colorless or near colorless state to a colored state. Upon removal of the potential difference between the electrodes, the electrochemically activated redox states of electroactive compounds react, becoming colorless again, and "clearing" the window.
Many improvements to electrochromic devices have been made. For example, use of a gel as a component of the electrochromic medium, as disclosed in U.S. Pat. Nos. 5,679,283 and 5,888,431, both entitled "Electrochromic Layer and Devices Comprising Same", and U.S. application Ser. No. 08/616,967, entitled "Improved Electrochromic Layer And Devices Comprising Same", now U.S. Pat. No. 5,928,572, have allowed the preparation of larger devices which are also less subject to hydrostatic pressure.
In electrochromic mirrors, devices are constructed with a reflecting surface located on the outer surface of the substrate which is most remote from the incident light (i.e. the back surface of the mirror), or on the inner surface of the substrate most remote from the incident light. Thus, light striking the mirror passes through the front substrate and its inner transparent conductive layer, through the electrochromic medium contained in the cavity defined by the two substrates, and is reflected back from a reflective surface as described previously. Application of voltage across the inner conductive coatings results in a change of the light reflectance of the mirror.
In electrochromic devices, the selection of the components of the electrochromic medium is critical. The medium must be capable of reversible color changes over a life cycle of many years, including cases where the device is subject to high temperatures as well as exposure to ultraviolet light. Thus, the industry constantly seeks new electrochromic media and new electroactive compounds which will resist aging, particularly in exterior locations. The effects of ultraviolet light, in particular, are felt more strongly when the electroactive compounds contained in electrochromic media are energized to their respective oxidized and reduced states.
In addition to problems associated with device stability over time, devices of improved color have been desired. The colors heretofore obtainable were limited both by the stability of available electrochromic compounds as well as economic factors such as their commercial availability and expense. Moreover, in many applications, for example electrochromic mirrors, it is desirable that the mirror, both in its inactive as well as its active state, be a relatively neutral color, for example gray. In addition, it is desirable that the color can be maintained over a range of voltage, for example, that the absorbance of the electrochromic medium may be changed without undesirably changing the hue, in particular between "full dark" and "clear" conditions.
Prior art electrochromic media generally employed two electrochromic compounds, one anodic and one cathodic, and were unable to acceptably produce gray shades, and numerous other shades of color as well. In U.S. application Ser. No. 08/832,596 filed Apr. 2, 1997, now U.S. Pat. No. 6,020,987 herein incorporated by reference, non-staging devices capable of achieving a preselected color are disclosed. These devices contain at least three active materials, at least two of which are electrochromic compounds, and exhibit little or no staging while being available in neutral colors such as gray, or in other preselected colors not normally available.
In accordance with the '596 disclosure, when two or more anodic electrochromic compounds are used, the redox potentials of the anodic electrochromic compounds should be relatively closely matched to prevent staging. The same is true if the electrochromic medium contains two or more cathodic electrochromic compounds; the redox potential of these should be relatively closely matched as well. This required close matching of redox potentials limits the choices available for electrochromic compounds. Thus, many electrochromic compounds which are relatively less expensive or which exhibit greater stability cannot be used when certain target colors and/or devices are envisioned because their redox potentials are unsuitable and prevent their being used in an electrochromic device.
It would be desirable to prepare electrochromic devices which are highly stable, which allow for a wide choice of electroactive compounds, and which are capable of being manufactured in a wide range of preselected colors, all at commercially acceptable cost.